Various bias circuits are known in the prior art (see Japanese Laid-open Patent Publication 2000-174568, Japanese Laid-open Patent Publication 2000-278053, and so on, for example).
FIG. 6 depicts a constitutional example of an amplifier circuit 100 and a Gm compensation bias circuit 120 according to the prior art. A gain of the amplifier circuit 100 is expressed as a product (R×Gm) of a resistance value R of a resistor 101 and a transconductance (mutual conductance) Gm of a transistor 102.
However, the resistor 101 and the transistor 102 are different elements, and since the transconductance Gm and the resistance R are not mutually related, they are affected independently by manufacturing conditions and temperature variation. As a result, in the case of not using the GM compensation circuit, the gain of the amplifier circuit 100 is not stable.
Hence, a Gm compensation bias circuit 120 outputs a Gm compensation bias voltage to the transistor 100 such that the transconductance Gm is inversely proportionate to the resistance R (Gm∝1/R), whereby the gain of the amplifier circuit 100 can be made constant (R×Gm∝R×1/R=constant).
Here, an output direct current voltage (DC voltage) output from an output terminal OUT of the amplifier circuit 100 can be expressed byVdd−R×Igm   (Equation 1),where Vdd is a power supply voltage and Igm is a drain current flowing into the transistor 102.
However, even when the gain of the amplifier circuit 100 can be made constant by the Gm compensation bias circuit 120, if the drain current Igm varies, the output direct current voltage of the amplifier circuit 100 varies greatly, as depicted in Equation 1. The drain current Igm is a current at which the transconductance of the transistor 102 is inversely proportionate to the resistance value, and this current is affected greatly by the physical characteristics of the transistor 102 as well as manufacturing and temperature variation, causing it to vary irrespective of the resistance value. FIG. 7 depicts an example of the manner in which the output direct current voltage varies greatly (indicated by broken lines in the drawing).
As a result of this large variation in the output voltage of the amplifier circuit 100, a distortion characteristic (compression characteristic) of an output signal deteriorates. For example, when the output current voltage is close to the power supply voltage, the output signal is limited by the power supply voltage, and when the output current voltage is close to the ground, the output signal is limited by the ground potential.